ABSTRACT Biological processes are controlled by multiple genes working in concert to achieve a given function. This phenomenon is apparent in genetic interactions, defined as a phenotype observed in a double mutant not easily explained by the phenotypes in the respective single mutants. While genetic interactions have long been recognized as important drivers of animal phenotypes, it has not been possible to perform genetic interaction analysis in animals in a systematic, null allele, reverse-genetics fashion. This is a critical gap, because understanding healthy and disease states in animals requires an appreciation of how multiple genes coordinately affect a given phenotype. To overcome this gap, we have developed a CRISPR/Cas9 toolkit that enables targeted genome modification and subsequent genetic interaction analysis in the nematode worm Caenorhabditis elegans, thus enabling for the first time systematic targeted genetic interaction profiling in animals. We will focus on genetic interactions among factors regulating gene expression. Proper gene expression is controlled by multiple layers of regulation (e.g. transcription, RNA processing, translation) but little is known about how these layers are coordinated at the level of single cells. The first direction of the lab therefore is to profile genetic interactions between different layers of gene expression, specifically focusing on transcription factors (TFs) and RNA binding proteins (RBPs). Double mutant combinations with unexpected phenotypes will be the entry point to mechanistic understanding of how combinations of TFs and RBPs coordinately control gene expression. The second direction of the lab will be to understand the regulation of alternative splicing at the single cell level by combinations of TFs and RBPs. Individual cell types can be defined by the presence of TFs and the resulting gene expression patterns, but can also be further refined by the presence of splicing factors and the resulting isoforms expressed. We have created a large number of in vivo splicing reporters in C. elegans and found extensive alternative splicing at the single cell level. Using a combination of forward and reverse genetics we have identified a number of splicing factors, as well as a surprising number of TFs, important for specific alternative splicing regimes at the single cell level. We now plan to investigate the mechanisms by which these factors combine to control splicing at the single cell level, as well as the functional consequences of such splicing. Together these directions will represent a key advance in our understanding of combinatorial action of gene regulatory factors and how they coordinately ensure proper gene expression.